Native to the planet Kessae, the Koepisi were the cause of a local Imperial Lord's fall to chaos. Starlisi raiders, riding the titular Koepisi, harassed trade caravans among loyal Imperial cities causing one Lord to turn to chaos to defeat them. They have an entry in the famed Calixis Black Grimoire as follows:

Koepis
Kessae Magnus Atrox
Homeworld: Kessae, Malfian Sub-Sector, Calixis Sector, Segmentum Obscurus
Description: The Koepis (pl. Koepisi) is a large, non-sentient beast native to
the planet of Kessae. Individuals may grow up to 4 metres in height at the
shoulder and 8 metres long from nose to tail. Weight is estimated to be between
8,000 and 10,000 kilograms.
Specimens observed appear to be solidly built, with powerful forelimbs and
jaws. Hindquarters are less developed, but still able to propel the beast at
speed and support the beast's weight when standing on hindquarters. Limbs carry
large claws strong enough to burrow through the rocky ground of Kessae.
Contact with beasts shows they have a natural resilience to las-fire, possibly
due to thick, coarse fur and/or a theorised layer of storage fat. Beasts also
exhibit an agility unusual for their bulk. Local information also suggests a
heightened sense of smell.
Behaviour: Highly territorial in the wild, Koepisi are quick to aggress.
However, indigenous populations known as "Nomads" have been observed riding the
creatures, indicating an ability to domesticate beasts. Attempts to contact
Nomads have been met with same territoriality exhibited by wild Koepisi. If the
Nomads will meet with representatives of the Imperium, large-scale use of the
beasts could prove highly beneficial.
Threat Rating: Xenos Majoris

"Its those damned beasts. The starlisi aren't much of a problem, however the koepisi are nigh-unstoppable. I still don't know how those nomads are able to control such wild beasts. I've heard rumours of magic, but I doubt savages such as those would be able to harness such power." - Lord Urser

He drops the rest of his magazine into the dazed creature, wondering if it took half an Inquisitorial kill-team to take it down, what the hell a koepisi was gonna do. - Hughes on fighting a warp abomination used to fight Koepisi